In the past 50 years, lithography has been dominated the micro/nano processing technology. Since the conventional single-photon lithography is achieved on basis of its planar nature, the processing resolution is restricted by the optical diffraction limit. In order to achieve a higher resolution, the lithography uses the wavelength of the laser output from the light source from infrared laser to DUV KrF laser (248 nm) and ArF laser (193 nm), with processing method from the traditional laser lithography, to X-ray lithography, electron-beam lithography, ion-beam lithography, nanopattern transfer, and so on. These technologies may prepare two-dimensional or quasi-three-dimensional structures by plane method, probe method or modeling method. In order to obtain a higher accuracy and finer feature size, the costs for manufacturing and maintaining the apparatus are very high. Meanwhile it is difficult for the conventional technologies to obtain a micro/nano device with complicated structures. Therefore, there is a need for a flexible, efficient, and low cost method for fabricating a micro/nano device with high resolution and high accuracy.
Direct femtosecond laser writing is a new super-fine processing technology developed in these years. Femtosecond laser lithography confines the area of the reaction between the laser and the substance to a small scope near the focal point by the non-linear optical effect and two-photon absorption effect, and thus may achieve the three-dimensional fabrication with the resolution exceeding the diffraction limit and obtain the micro/nano device with the nano-scaled feature size. Femtosecond laser lithography has the beneficial effects of high accuracy, truly three-dimension and low cost. In 2011, Professor Satoshi Kawata et al. achieved a processing resolution of 120 nm with the negative photoresist SCR500 on the glass substrate by pulsed femtosecond laser at wavelength λ of 780 nm, and prepared a three-dimensional nano-bull structure, see Satoshi Kawata et al., “Finer features for functional microdevices”, Nature 2001, 412 (6848), 697-698. In 2008, Xian-Zi Dong et al. at Technical Institute of Physics and Chemistry, CAS, achieved a resolution of 50 nm on the glass substrate with the negative photoresist SCR500 using a pulsed femtosecond laser at wavelength of 780 nm by controlling the parameter of the laser, see Xian-Zi Dong et al., “Improving spatial resolution and reducing aspect ratio in multiphoton polymerization nanofabrication”, Appl. Phys. Lett., 2008, 92: 091113. In addition, in 2007, Dengfeng Tan et al. at Beijing University achieved a suspended polymer nanolines with line width of 15 nm between the pre-formed cuboids by polymerization shrinkage effect, see Dengfeng Tan et al. “Reduction in feature size of two-photon polymerization using SCR500”, Appl. Phys. Lett., 2007, 90: 071106. For the positive photoresist, in 2005, B. N. Chichkov, et al. achieved a processing resolution of 140 nm by using a diluted photoresist (g-line S1813, Shipley Company LLC), and prepared a three-dimensional hollow wood-pile structure, see Claude Phipps, “Laser ablation and its applications”, Springer. 2007, 141-142.
Some scientists proposed to use two laser beams to further improve the resolution, one for inducing the photopolymerization reaction and another for confining the reaction with the material only at the center of the focal point of the excitation laser for initiating, which significantly breaks through the diffraction limit. Timothy F. Scott et al. achieved to confine the reaction area to a very small area of the focal point of the exciting light and obtained a processing resolution smaller than the diffraction limit by using one laser beam from all-solid-state laser at a wavelength of 473 nm to excite the free radical to induce photopolymerization, and using another laser beam from Ar ion laser at a wavelength of 365 nm to dissipate the free radicals around the focal point of the excitation laser, see Timothy F. Scott et al. Science, 2009, 324 (5929), 913. Linjie Li et al. achieved a longitudinal resolution of 40 nm by using near infrared laser at a wavelength of 800 nm, pulse width of 200 fs from the femtosecond laser to induce the material polymerization through a two-photon absorption, and using another pulsed laser at the same wavelength, pulse width of 50 ps to depleting the free radicals so as to control the degree and extent of the polymerization near the focal point of the exciting light, see Linjie Li, et al. Science, 2009, 324 (5929), 910. Trisha L. Andrew et al. tried to cover on the photoresist a photochromic film, which passes through a laser at wavelength of 325 nm and absorbs the laser at wavelength of 325 nm at the reaction area under the laser at wavelength of 633 nm from He—Ne laser, and use Lloyd's-mirror interferometer to interfere the two lasers and have alternate bright and dark interfere stripes, such that the laser at wavelength of 325 nm passed through the photochromic film in extremely small area reacts with the photoresist to achieve a lateral processing resolution of 36 nm, see Trisha L. Andrew et al., Science, 2009, 324 (5929), 917. However, the above mentioned methods are only be applicable to the materials that can be excited by laser and their excited state may be depleted, and are not applicable to other materials.
Furthermore, some scientists started to use femtosecond laser to induce the multiphoton reduction of metal ions to achieve micro/nano structures. In 2000, Pu-Wei Wu et al., focused the femtosecond laser into the metal ion doped transparent silica gel to reduce the metal ion to metal atom by charge transfer procedure from the excited state of silica gel having absorbed multiphotons to the noble metal ion, and thus prepared a three-dimensional micron helix, see Pu-Wei Wu et al., Two-Photon photographic production of three-dimensional metallic structures within a dielectric matrix, Advanced Materials, 2000, 12 (19): 1438-1441. In 2006, Takuo Tanaka et al. at RIKEN, Japan, prepared a tilt column and a bowl by directly reducing the silver ion in the AgNO3 aqueous solution with femtosecond laser and obtained a silver line of 400 nm with the resistivity as 3.3 times as the resistivity of the bulk silver, see Takuo Tanaka et al., Two-photo-induced reduction of metal ions for fabricating three-dimensional electrically conductive metallic microstructure, Applied Physics Letters, 2006, 88: 081107. In 2008, Shoji Maruo et al. prepared a silver line in the polymer by reducing the silver ion in polyvinylpyrrolidone with the femtosecond laser. By adjusting the concentration of the silver ion in the polymer, the resistivity of the silver line can be lowered down to as about 2 times as that of the bulk silver, see Shoji Maruo et al., Femtosecond laser direct writing of metallic microstructures by photoreduction of silver nitrate in a polymer matrix, Optics Express, 2008, 16(2): 1174-1179. In 2009, Yao-Yu Cao et al. at Technical Institute of Physics and Chemistry, CAS achieved a silver line with the smoother surface topography by adding surfactant NDSS into ammoniacal silver solution and controlling the size of silver nanoparticles during the femtosecond laser reduction, the tomography and resolution of the silver line are significantly improved, see Yao-Yu Cao et al., 3D metallic Nanostructure Fabrication by surfactant-assisted multiphoton-induced reduction, Small, 2009, 5(10): 1144-1148. In 2010, Bin-Bin Xu at Jilin University prepared a silver line by adding Sodium Citrate into ammoniacal silver solution, which achieved a silver line of 125 nm with its resistivity as about 10 times as that of the bulk silver, see Bin-Bin Xu et al., Flexibly metal nanowiring on nonplanar substrates by femtosecond laser induced electroless plating, Small, 2010, 6(16): 1762-1766. However, currently fabrication of the metal micro/nano structure with femtosecond laser may achieve the resolution more than one hundred nanos, it is difficult to achieve a metal structure of nano-scale.
Therefore, there is a need to provide a laser micro/nano processing system and processing method which scans the material to be processed with the laser at wavelength matching the optical absorptive characteristics of the material so as to precisely control the resolution and the accuracy.
Normally, the glass sheet applied with the photoresist is placed on the micromovement stage of the femtosecond laser fabrication apparatus. The incident laser beam passes through the oil immersion objective and irradiates the photoresist from the bottom of the glass so as to achieve a small focal spot and the improved resolution due to the matching refractive indexes of the oil and the glass, and to protect the objective lens from damaging. Therefore, the conventional femtosecond laser technology is limited to process the photosensitive material formed on the glass substrate. Therefore, there is a need to provide a laser two-photon direct-writing technology which may process the photoresist formed on various types of substrates and may be applied in various application fields.